The present invention relates to coated metallic articles exhibiting low hydrogen embrittlement, in addition to such articles that exhibit high corrosion resistance, and most particularly to metallic articles in which such coatings are created by electrodeposition techniques.
Steel, and especially high strength steel, is subject to delayed, brittle failures at relatively low stresses. Such failures have been attributed to the presence of hydrogen in the steel microstructure. The hydrogen can be introduced to the structure by reaction with water or with an acid, or by cathodically discharging hydrogen at the surface of the steel. Most high strength steels have corrosion resistant metallic coatings that are applied by electroplating techniques. Because these techniques are less than 100 percent efficient, hydrogen is usually discharged onto the steel surface along with the corrosion resistant coating. In order to minimize embrittlement caused by the presence of excess hydrogen, the quantity of hydrogen deposited throughout the coated article must be carefully monitored and controlled and, if necessary, altered to produce an article that exhibits low hydrogen embrittlement.
Currently, cadmium-titanium alloys are electroplated onto high strength steels under carefully controlled conditions to produce a steel product that is corrosion resistant. The resulting plated product is then heated at elevated temperatures to drive off hydrogen and achieve a structurally acceptable product. It is believed that the porosity of the electroplated cadmium-titanium alloy is the key to the removal of the hydrogen during this heat treatment. However, the cadmium-titanium plating bath is very sensitive to contamination. When the plating bath is contaminated, the resulting product is much more susceptible to embrittlement because the coating porosity is affected. Additionally, the concentration of the various components in the cadmium-titanium plating bath must be carefully controlled within very precise ranges to achieve acceptable hydrogen embrittlement and corrosion resistance characteristics at the same time. Moreover, expensive waste treatment must be performed on the spent plating bath before the components of the bath can be safely discharged into the sewage system.
As a consequence, alternatives to the cadium-titanium plating process have been sought that will provide low corrosion and low hydrogen embrittlement characteristics and be less expensive as well as easier to control. Zinc-nickel alloys are known to provide corrosion resistant electrodeposited coatings. Zinc-nickel alloys have also been employed where high speed plating processes are used. High speed plating is not satisfactory for rack plating of larger metal parts. Prior attempts at plating zinc-nickel alloy at low current densities have led to pitted and spongy deposits that do not provide good corrosion resistance. Moreover, prior attempts to produce a zinc-nickel corrosion resistant coating has led to a relatively high degree of hydrogen embrittlement in plated high strength steel parts to which the coating has been applied. Although prior work with zinc-nickel plating baths had not successfully produced a zinc-nickel coating that would provide both low hydrogen embrittlement and acceptable corrosion resistanct characteristics, the attractiveness and ease with which zinc-nickel baths could be controlled and adjusted resulted in further research. As a result of this research, a method for plating a zinc-nickel coating that would achieve both low hydrogen embrittlement and high corrosion resistance characteristics was developed. This process is disclosed in prior copending application Ser. No. 348,107, filed Dec. 28, 1981, now abandoned, entitled Zinc-Nickel Electroplated Article and Method for Producing the Same. The process disclosed and claimed in that application requires that the zinc-nickel coating be electrodeposited in the presence of a soluble ammonium salt and other additives.